The present invention relates to a flow sensor for detecting the flow amount and flow rate of a fluid, a method of manufacturing the same and a fuel cell system using the flow sensor.
FIGS. 1 and 2 are conceptual diagrams of a flow sensor 1 having a conventional structure. FIG. 2 is a sectional view taken along the line X1xe2x80x94X1 in FIG. 1. FIG. 1 shows a heater and a temperature measuring element in an exposed state, and FIG. 2 shows the same members covered with a protective film 10 or the like. In the flow sensor 1, a gap section 3 in the form of a recess is formed on a top surface of a silicon substrate 2; an insulating thin film 4 is provided on the top surface of the silicon substrate 2 such that it covers the gap section 3; and a part of the insulating thin film 4 forms a bridge section 5 in the form of a thin film over the gap section 3. The bridge section 5 is thermally insulated from the silicon substrate 2 by the space (air) in the gap section 3. A heater 6 is provided in the middle of a surface of the bridge section 5, and temperature measuring elements 7 and 8 are provided in respective positions which are symmetrical about the heater 6. An ambient temperature measuring resistive element 9 is provided on a surface of the insulating thin film 4 located outside the bridge section 5. Further, the silicon substrate 2 is coated with a protective film 10 such that the heater 6, temperature measuring elements 7 and 8, and ambient temperature measuring resistive element 9 are covered with the same.
Various elements are used as the temperature measuring elements 7 and 8. For example, Japanese unexamined patent publication No. S60-142268 disclosed the use of thin film resistors made of an iron-nickel alloy. In an article titled, xe2x80x9cLow power consumption thermal gas-flow sensor based on thermopiles of highly effective thermoelectric materialsxe2x80x9d, is disclosed the use of BiSbxe2x80x94Sb thermopiles as temperature measuring elements. Further, transistors or the like may be used as temperature measuring elements. The following description is based on an assumption that thermopiles formed from BiSbxe2x80x94Sb thermocouples are used as the temperature measuring elements 7 and 8.
When thermopiles formed from BiSbxe2x80x94Sb thermocouples are used as the temperature measuring elements 7 and 8, thin wires made of BiSb and thin wires made of Sb are alternately provided across edges of the bridge section to form a group of hot contacts 11 at points where the BiSb thin wires and Sb thin wires are connected in the bridge section 5, and to form a group of cool contacts 12 at points where the BiSb thin wires and Sb thin wires are connected outside the bridge section 5.
A voltage V1 output by the temperature measuring element 7 (a voltage across the same) and a voltage V2 output by the temperature measuring element 8 (a voltage across the same) are respectively expressed by Equations 1 and 2 as follows where n represents the quantities of the hot contacts 11 and cool contacts 12 of the temperature measuring elements (thermopiles) 7 and 8; Tc represents the temperature of the cool contacts 12 (which is equivalent to the ambient temperature at the time of measurement); Th1 represents the temperature of the hot contacts of the temperature measuring element 7; and Th2 represents the temperature of the hot contacts 11 of the temperature measuring element 8.
V1=nxc2x7a(Th1xe2x88x92Tc)xe2x80x83xe2x80x83Equation 1
V2=nxc2x7a(Th2xe2x88x92Tc)xe2x80x83xe2x80x83Equation 2
xe2x80x9caxe2x80x9d represents a Seebeck coefficient.
The flow sensor 1 is placed in a channel 13 through which a fluid flows as shown in FIG. 3, and the outputs of the temperature measuring elements 7 and 8 are monitored with the heater 6 heated by a current applied thereto. When there is no wind, or when no gas flows, since the temperature distribution on the surface of the insulating thin film 4 is symmetric about the heater 6, as indicated by the solid line in FIG. 5, the temperature Th1 of the hot contacts of the temperature measuring element 7 and the temperature Th2 of the hot contacts of the temperature measuring element 8 are equal to each other because of the symmetry of their positions, and the voltage V1 output by the temperature measuring element 7 and the voltage V2 output by the temperature measuring element 8 are therefore equal to each other.
On the contrary, when a fluid flows from the temperature measuring element 7 toward the temperature measuring element 8 as indicated by the arrow in FIG. 4, the temperature distribution on the surface of the insulating thin film 4 is asymmetric, as indicated by the broken line in FIG. 5. Specifically, the temperature Th1 of the hot contacts of the temperature measuring element 7 located upstream decreases because the element is cooled by the flow of the fluid, and the output voltage V1=nxc2x7a(Th1xe2x88x92Tc) decreases. Meanwhile, the heat of the heater 6 is transported by the fluid downstream to increase the temperature Th2 of the hot contacts of the temperature measuring element 8 located downstream, which results in an increase in the output voltage V2=nxc2x7a(Th2xe2x88x92Tc). The flow amount of the fluid can be measured from a resultant change xcex94V=V2xe2x88x92V1 in the output voltage. When the flow amount of the fluid is small, since the difference xcex94T=Th2xe2x88x92Th1 between the temperatures of the temperature measuring elements 7 and 8 is proportionate to the mass flow of the fluid, the temperature difference can be obtained from Equation 3 shown below by measuring the output voltages V1 and V2 of the temperature measuring elements 7 and 8, and the mass flow of the fluid can be calculated by performing further calculation processes that are required.                                                                         Δ                ⁢                                  xe2x80x83                                ⁢                T                            =                              Δ                ⁢                                  xe2x80x83                                ⁢                                  V                  /                                      (                                          n                      ·                      a                                        )                                                                                                                          =                                                (                                      V2                    -                    V1                                    )                                /                                  (                                      n                    ·                    a                                    )                                                                                        Equation        ⁢                  xe2x80x83                ⁢        3            
The ambient temperature measuring resistive element 9 measures the ambient temperature of the flow sensor 1. The ambient temperature is measured with the ambient temperature measuring resistive element 9 to maintain the heating temperature of the heater 6 at a temperature which is higher than the ambient temperature by a constant value at any flow rate (this effect is hereinafter referred to as xe2x80x9cconstant temperature rise of the heater 6xe2x80x9d) and to correct temperature characteristics of the flow sensor 1.
In the flow sensor 1, when the heating temperature of the heater 6 increases, the output voltages of the temperature measuring elements 7 and 8 increase in proportionate to the same, which improves the resolution of a temperature measured by the temperature measuring elements 7 and 8. The higher the heating temperature of the heater 6, the greater the power consumption of the heater 6. Therefore, the heating temperature of the heater 6 is set by a user at an arbitrary constant temperature taking both factors into consideration.
However, the heating temperature of the heater 6 changes depending on the flow rate of a fluid. In an environment in which the flow sensor 1 is used, the ambient temperature normally changes. For those reasons, a change in the difference between the ambient temperature and the heating temperature of the heater 6 results in a change in a temperature gradient around the heater 6 and a change in the relationship between the output voltages of the temperature measuring elements 7 and 8 and the quantity or rate of the flow of a fluid, which deteriorates the accuracy of measurement.
Under such circumstances, a heater control circuit 14 as shown in FIG. 6 is used in the conventional flow sensor to automatically adjust the heating temperature of the heater 6 to a temperature which is higher than the ambient temperature detected by the ambient temperature measuring resistive element 9 by a constant value (a constant temperature rise of the heater). The heater control circuit 14 is comprised of fixed resistors 17 and 18, voltage-dividing resistors 19 and 20, an operational amplifier (differential amplifier circuit) 15, and a transistor 16. The fixed resistors 17 and 18 formed a bridge circuit in combination with the heater 6 and the ambient temperature measuring resistive element 9. A mid-point between the fixed resistor 17 and the ambient temperature measuring resistive element 9 is connected to an inverting input terminal of the operational amplifier 15, and a mid-point between the fixed resistor 18 and the heater 6 is connected to a non-inverting input terminal of the operational amplifier 15. The transistor 16 is inserted between a power supply Vcc and the fixed resistor 17, and series-connected voltage-dividing resistors 19 and 20 are connected between the base of the transistor 16 and the ground. The output of the operational amplifier 15 is connected to a mid-point between the voltage-dividing resistors 19 and 20.
The heater control circuit 14 is intended to keep the heater 6 in a thermal equilibrium state at a temperature which is higher than the ambient temperature by a constant value. When the temperature of the heater 6 decreases as a result of transition from the thermal equilibrium state e.g., a no-wind state to a state in which there is a flow of a gas, the potential at the non-inverting input terminal of the operational amplifier decreases to drive the transistor 16 which in turn supplies a current to restore the thermal equilibrium state, which operation occurs in a repetitive manner. A similar operation occurs when there is a change in the ambient temperature. Specifically, in the heater control circuit 14, when the heating temperature of the heater 6 increases beyond the temperature of the same in the equilibrium state, a voltage at the mid-point between the voltage-dividing resistors 19 and 20 increases because there is an increase in the current output by the operational amplifier 15. As a result, the base current of the transistor 16 decreases to decrease the current flowing through the bridge circuit. Consequently, the current flowing through the heater 6 decreases to reduce the heating temperature of the heater 6. When the heating temperature of the heater 6 conversely decreases below the temperature of the same in the equilibrium state, since the current output by the operational amplifier 15 decreases, the voltage at the mid-point between the voltage-dividing resistors 19 and 20 decreases. This results in an increase in the base current of the transistor 16 and in an increase in the current flowing through the bridge circuit too. Consequently, the current flowing through the heater 6 increases to increase the heating temperature of the heater 6.
When the temperature of the ambient temperature measuring resistive element 9 for detecting the ambient temperature increases beyond the temperature in the equilibrium state, since the current output by the operational amplifier 15 decreases, the voltage at the mid-point between the voltage-dividing resistors 19 and 20 decreases. This results in an increase in the base current of the transistor 16 and an increase in the current flowing through the bridge circuit. This increases the current flowing through the heater 6 to increase the heating temperature of the heater 6. When the heating temperature of the ambient temperature measuring resistive element 9 conversely decreases below the temperature in the equilibrium state, since there is an increase in the current output by the operational amplifier 15, the voltage at the mid-point between the voltage-dividing resistors 19 and 20 increases. As a result, the base current of the transistor 16 decreases, which in turn decreases the current flowing through the bridge circuit. Consequently, the current flowing through the heater 6 decreases to reduce the heating temperature of the heater 6.
As thus described, the heater control circuit 14 operates to keep the resistance of the heater 6 constant and automatically adjusts the heating temperature of the heater 6 to a constant value.
The resistance of the ambient temperature measuring resistive element 9 is input to a temperature correction circuit including a CPU and having a calculating function, and the difference xcex94V=V2xe2x88x92V1 between the output voltages of the temperature measuring elements 7 and 8 are corrected based on a change in the ambient temperature detected by the ambient temperature measuring resistive element 9 to correct temperature characteristics. Therefore, a separate temperature correction circuit has been required for the conventional flow sensor in order to make a correction in accordance with the ambient temperature.
It is an object of the invention to improve temperature characteristics (dependence on ambient temperature) of a flow sensor and to eliminate any need for a temperature correction circuit which has been required in the prior art.
A flow sensor according to the invention is a flow sensor comprising a substrate, an insulation layer formed as a thin film on a surface of the substrate, a heating element provided on a surface of the insulation layer, at least one temperature measuring element provided on at least one side of the heating element on the surface of the insulation layer, and a gap formed at the semiconductor substrate under at least parts of the heating element and temperature measuring element, characterized in that the temperature measuring element has ambient temperature dependence which cancels ambient temperature dependence attributable to factors other than the temperature measuring element. The term xe2x80x9cambient temperature dependencexe2x80x9d in this context means a change in the output of a flow sensor attributable to a change in the ambient temperature of the flow sensor.
Since the temperature measuring element of the flow sensor according to the invention has ambient temperature dependence which cancels ambient temperature dependence attributable to factors other than the temperature measuring element, the flow sensor has a whole has small ambient temperature dependence which is a combination of the ambient temperature dependence of the temperature measuring element and the ambient temperature dependence attributable to factors other than the temperature measuring element. This eliminates any need for a temperature correction circuit for correcting ambient temperature dependence unlike the prior art.
In a mode for carrying out the invention intended for cancellation of ambient temperature dependence of a flow sensor between a temperature measuring element and factors other than the temperature measuring element, the temperature measuring element may be constituted by a thermopile, and a configuration may used in which the gap is provided under a hot contact to keep ambient temperature dependence of the temperature measuring element at a constant ratio to ambient temperature dependence attributable to factors other than the temperature measuring element. By keeping the ambient temperature measuring element at a constant ratio to the ambient temperature dependence attributable to factors other than the temperature measuring element, the ambient temperature dependence can be easily canceled within the temperature range of the same. Especially, in a configuration in which ambient temperature dependence of a thermopile serving as a temperature measuring element has an absolute value of ambient temperature dependence substantially equal to that of ambient temperature dependence attributable to factors other than the thermopile, ambient temperature dependence of a flow sensor can be suppressed with high accuracy.
In another mode for carrying out a flow sensor according to the invention, the temperature measuring element is constituted by a thermopile; the gap is provided under a hot contact; at least a part of a material that makes up the thermopile is doped with a dopant, which provides the thermopile with ambient temperature dependence that cancels ambient temperature dependence attributable to factors other than the thermopile. Since a change in the dose for the doping of the thermopile results in a change in the ambient temperature dependence of the thermopile, the dose may be adjusted in consideration to the ambient temperature dependence attributable to factors other than the thermopile to cancel the ambient temperature dependence attributable to factors other than the thermopile with the ambient temperature dependence of the thermopile.
In still another mode for carrying out a flow sensor according to the invention, the temperature measuring elements are provided on both sides of the heating element and spaced by the same, and the gap opens on a surface of the semiconductor substrate in a region between the temperature measuring elements. When the temperature measuring elements are thus provided on both sides of the heating element, the temperature of the temperature measuring element located upstream of a flow of a fluid decreases, and the temperature of the temperature measuring element located downstream increases. As a result, the sensitivity of the flow sensor can be improved by obtaining the difference between the temperatures of the two temperature measuring elements.
In still another mode for carrying out a flow sensor according to the invention, the thermopile is made of polysilicon and aluminum; phosphorus (P) is used as the dopant for controlling the ambient temperature dependence of the thermopile; and the dose for the dopant is in a range from 1.0xc3x971017 to 1.0xc3x971021 ions/cm3. An error of a temperature detected by the temperature measuring element can be kept in a range of xc2x10.1%/xc2x0 C. by keeping the dose of phosphorus within the range, which makes it possible to satisfy general specifications for temperature characteristics required for a flow sensor.
A method of manufacturing a flow sensor according to the invention is a method of manufacturing a flow sensor having a semiconductor substrate, an insulation layer formed as a thin film on a surface of the semiconductor substrate, a heating element provided on a surface of the insulation layer, thermopiles provided on both sides of the heating element and spaced by the same on a surface of the insulation layer, and a gap formed at the semiconductor substrate under regions of the thermopiles extending bonding portions at ends thereof and to the heating element, characterized in that a semiconductor material such as polysilicon is used as at least a part of the material serving as the thermopiles and in that ambient temperature dependence of the thermopiles is kept at a constant ratio to ambient temperature dependence attributable to factors other than the thermopiles by controlling the dose of an impurity with which the semiconductor material is doped.
In the method of manufacturing a flow sensor according to the invention, the ambient temperature dependence of the thermopiles is kept at a constant ratio to the ambient temperature dependence attributable to factors other than the thermopiles by controlling the dose of an impurity with which the semiconductor material for the thermopiles is doped, the flow sensor as a whole has small ambient temperature dependence which is a combination of the ambient temperature dependence of the thermopiles and the ambient temperature dependence attributable to factors other than the thermopiles. This eliminates any need for a temperature correction circuit for correcting ambient temperature dependence unlike the prior art. It is also possible to cancel ambient temperature dependence of the flow sensor as a whole easily by controlling the dose. Further, since the thermopiles are provided on both sides of the heating element, the temperature of the thermopile located upstream of a flow of a fluid decreases, and the temperature of the thermopile located downstream increases. Therefore, the sensitivity of the flow sensor can be improved by obtaining the difference between the temperatures of the tow thermopiles.
The above-described constituent elements of the invention may be used in any possible combination.